[tests] remove double-substitution in assert_ok and assert_not_ok

Co-authored-by: Андрій Шовкошитний <198119344+enlightened88@users.noreply.github.com>
Co-authored-by: Jordan Harband <ljharb@gmail.com>

test/fast/Unit tests/assert_ok

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+die () { echo "$@" ; exit 1; }
+
+\. ../../common.sh
+
+_returns_zero() { return 0; }
+_returns_nonzero() { return 1; }
+
+# assert_ok should pass for a zero-exit function
+assert_ok _returns_zero \
+  || die 'assert_ok failed on a function that returns 0'
+
+# assert_ok should fail for a non-zero-exit function
+if (assert_ok _returns_nonzero 2>/dev/null); then
+  die 'assert_ok incorrectly passed for a function that returns 1'
+fi